Three-dimensional integrated circuits (3DIC) and stacked dies are commonly used to decrease the overall size of integrated circuits. 3DICs and stacked dies use through-substrate vias (TSVs, also referred to sometimes as through silicon vias or through vias) to connect an individual die through its backside to another die. By using vertical space, 3DICs and stacked dies are able to reduce the overall footprint of an integrated circuit. Additionally, TSVs are used to create short grounding paths through the backside of a die, which is typically covered with grounded aluminum film. However, as improvements in technology continually decrease the size of integrated circuits and its constituent components, controlling the topography of the layers in a die becomes increasingly important.
In a typical die, a substrate comprising active devices, such as transistors, is provided, an inter-layer dielectric (ILD) is formed over the substrate, and then any number of inter-metal dielectrics (IMDs), comprising metallic trenches/vias used to execute logic functions, are formed over the ILD. In a 3DIC or stacked die, a TSV may also be formed at either the ILD or an IMD. The ILD acts as a barrier between the active devices in the substrate and the metallic trenches/vias in the IMD. The ILD prevents metallic particles in the IMD from diffusing into the substrate and is crucial to the proper functioning of the active devices in the substrate.
When formed at the ILD layer, the TSV may be covered with a liner and a metal pad in the subsequent IMD layer. The liner and pad act to create a flat top surface for stable connections to the TSV. Using this method, the ILD often suffers loss due to erosion and dishing caused by the TSV's formation process. ILD loss is especially problematic for advanced technologies where the die components are so compact and precise that tolerance for such loss is very limited. Furthermore, the use of a liner causes high contact resistance between the TSV and the overlying metal pad. On the other hand, formation of the TSV at the IMD layer suffers from issues of uneven topography due to TSV's size and grain growth. This uneven topography can lead to broken and unstable connections to the TSV.